1. Field of the Invention
The present invention relates to an ion flow electrostatic recording head used in electrostatic printing, copying, and the like, and a method for manufacturing the same.
2. Description of the Related Art
An electrostatic recording apparatus is generally used in, for example, electrostatic printing. This apparatus generates ions having a high current density by discharging high-frequency pulses. These ions are selectively supplied to a chargeable member, constituted by an insulator to charge the member, and then form a latent image thereon by an electrostatic charge. Powders (toners) are scattered on the member, which have an electrostatic charge whose polarity is opposite to that of the electrostatic charge of the latent image, thereby visualizing the latent image.
An ion flow electrostatic recording head used in the above electrostatic recording apparatus has the following structure. A plurality of first electrodes each formed of a long conductor are arranged in parallel on one surface of a sheet-like dielectric layer, and a plurality of second electrodes each formed of a long conductor are arranged on the other surface thereof. These first and second electrodes intersect each other to form a matrix. Ion generating holes are formed in those portions of the second electrodes which intersect the first electrodes.
An insulating layer is formed on the surfaces of the second electrodes which are opposite to the dielectric layer, and a sheet-like third electrode is formed on the insulating layer. A large number of ion current flowing openings are formed in the insulating layer and the third electrode. These ion current flowing openings are arranged so as to communicate with the holes formed in the second electrodes.
when the ion flow electrostatic recording head is operated, any one of the intersections between the first and second electrodes can be selected in response to a printing signal. A high voltage is applied between the first and second electrodes in correspondence with the selected intersection, with the result that positive and negative ions are generated in the vicinity of that hole of the second electrode which corresponds to the intersection.
Furthermore, a bias voltage is applied between the second and third electrodes. Only the ion, which depends upon the polarity of the bias voltage, is selected from the positive and negative ions generated near the opening of the second electrode. The selected ion is transmitted through the ion current flowing opening and then to a chargeable member which is spaced away from the third electrode, thereby partially charging this member. Consequently, electrostatic recording can be performed in the form of dots by selectively driving the electrodes constituting a matrix.
In general, a dielectric material for forming the dielectric layer necessitates such an insulating strength as not to cause a dielectric breakdown even when a high voltage is applied for ion generation. Since the dielectric layer requires such a thickness as to generate ions efficiently and withstand a dielectric breakdown, it is desirable that the dielectric layer should have a high dielectric constant.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2-153760 discloses an ion flow electrostatic recording head wherein a dielectric layer is formed by mixing a powder of titanium oxide into silicone denatured polyester alkyd resin. Further, the second electrodes, which are constituted by foil of stainless steel, are fixedly adhered to the dielectric layer using a silicone type pressure sensitive adhesive.
The process of manufacturing the ion flow electrostatic recording head includes a heating step and a mechanical folding step following the step of fixedly adhering the second electrodes to the dielectric layer. In the heating and folding steps, a thermal stress or a mechanical stress are applied to each of the components constituting the recording head. Also, while the recording head is being operated, heat generates from the whole of the head, and a thermal stress is applied to each of the components of the head. The thermal or mechanical stress is concentrated on the least cohesive portion of the components of the recording head, e.g., a fixing portion between the second electrode and the dielectric layer.
However, the silicone type pressure sensitive adhesive of Jpn. Pat. Appln. KOKAI Publication No. 2-153760 is strong in peeling but weak in shearing. For this reason, when the silicone type pressure sensitive adhesive is used to fix the second electrode to the dielectric layer, it does not withstand the shear force due to the thermal or mechanical stress generated in the heating step, in the folding step, or in using the recording head. Thus, the first and second electrodes may shift away from each other after the second electrode is fixed to the dielectric layer, resulting in the degradation of an image formed by the recording head.
The area of each second electrode contacting the surface of the dielectric layer is very small, only several square millimeters. It is thus necessary to use a special adhesive having a high adhesiveness in order to fix the second electrodes and the dielectric layer by only the adhesion of the adhesive without causing any shift between them and to withstand the thermal and mechanical stresses in the heating and folding steps and in using the recording head. The use of such an adhesive increases the manufacturing cost.
Since several thousands of small ion generating holes each having a diameter of about 100 .mu.m are formed in the second electrode, it is very difficult to apply an adhesive to portions other than the holes. Furthermore, an unnecessary adhesive has to be removed from inside the holes by washing the contact portions between the second electrodes and the dielectric layer, which is a complicated operation.
Since it is difficult to completely remove the adhesive attached to all the holes formed in the second electrodes using only a washing operation, the adhesive may remain locally inside the holes. In contrast, an indispensable adhesive may peel off the periphery of each of the holes of those sides of the second electrodes which are adhered to the dielectric layer and the metal surface may thus be exposed. Since the current density of the exposed metal surface, which contributes to discharge, differs from that of the portion covered with the adhesive, the amounts of ions generated from the holes differ, one from another, and it is difficult to form an image having uniform quality by using the recording head and to have uniform durability of all the second electrodes.
A high-level etching technique is required to uniformly make the numerous holes in the second electrode of stainless foil with high precision. In this respect, the manufacturing cost of the recording head is increased.